1. Field of the Invention
The present invention relates to an STM-N (Synchronous Transport Module Level N) signal error correction encoding/decoding method and STM-N signal error correction encoding/decoding circuits in use for the STM (Synchronous Transfer mode). The method and circuits may be used, for example, in error correction encoding/decoding in an SDH (Synchronous Digital Hierarchy) fiber optical transmission system.
2. Description of the Related Art
Recently, the SDH fiber optical transmission system has been remarkably researched. The following are some of the documents dealing with this technology:
Document 1: Yoshiaki YAMABAYASHI, Masahito TOMIZAWA, Tomoyoshi KATAOKA, Yukiko KOBAYASHI, and Kazuko HAGIMOTO. OAA" 94, ThA6: NTT "A Bit-Interleaved Hamming Code for Linearly Repeated Terrestrial Fiber Optic Transmission Systems" PA1 Document 2: Masaki Tomizawa, Yoshiaki Yamabayashi, Norio Kobayashi, and Kazuo Hagimoto. 1995. "Study on the Error Correction Code of an SDH Fiber Optical Transmission System," Electronic Information Communication National Convention B-1074.
The above documents propose the forward error correction code (FEC) compatible with the SDH. This forward error correction circuit, installed on a terminal equipment, has the check bits stored in the undefined byte area in the multiplex-section overhead (MSOH) shown in FIG. 2. In FIG. 2(b), "X" indicates a byte for an international standard code reserved for future use, ".DELTA." indicates a media-dependent byte, and * indicates a byte not scrambled.
The STM-N (1.ltoreq.N.ltoreq.16) signal is in one of the following two forms: one form is that various concatenation paths VC(virtual container)-4-Xc are mapped with the use of ATM (Asynchronous Transfer Mode) cells and the other form is that N AU-4 signals, each of which is a VC-4 to which the AU (Administrative Unit) pointer is added, are byte-multiplexed.
When error correction encoding is performed on AU-4-Xc of the STM-N signal, various concatenation paths are mapped, on an SDH terminal unit, into X AU-4 signals each of which is AU-4-Xc generated by adding the AU pointer to VC-4-Xc. Therefore, the error correction circuit performs encoding on AU-4, and X encoding circuits and X decoding circuits are used.
When N AU-4 signals, each of which has the AU pointer in VC-4, are byte-multiplexed, N STM-1 signals each containing AU-4, check bytes, and the SOH are byte-multiplexed. This method may be used for any STM-N system since the FEC encoding circuit encodes one AU-4 at a time. The following two types of code are provided for the FEC code of an SDH-compatible FEC encoding circuit.
(1) Parallel type shortened Hamming code: One-bit correction is performed on the eight parallel bits of AU-4 at a time (18864 bits/8=2358 bits). Twelve bytes are required for the check bits. When the encoding target is AU-4-Xc (2.ltoreq.X.ltoreq.16) or X (2.ltoreq.X.ltoreq.16) AU-4 signals, X encoding circuits and X decoding circuits are used.
(2) Serial type shortened Hamming code: One-bit correction is performed on one AU-4 at a time (18864 bits=241 columns.times.9 rows/1 column+{9 columns.times.1 row.times.8 bits). Two bytes are required for the check bits. When the encoding target is AU-4-Xc (2.ltoreq.X.ltoreq.16) or X (2.ltoreq.X.ltoreq.16) AU-4 signals, X encoding circuits and X decoding circuits are used.
However, the conventional error correction encoding/decoding method described above uses AU-4 as the encoding target for both the serial type and parallel type shortened Hamming codes. Therefore, when the shortened Hamming code is used for AU-4-Xc (2.ltoreq.X.ltoreq.16) or X (2.ltoreq.X.ltoreq.16) AU-4 signals in the STM-N (2.ltoreq.N.ltoreq.16) signal as the error correction code, X encoding circuits and X decoding circuits are required. And, the serial type shortened Hamming code requires 2.times.X bytes of check bits, and the parallel type shortened Hamming code requires 12.times.X bytes of check bits, showing that the required check bits are X times as many as the check bits of the AU-4 signal.
This causes the following two problems concerning larger value of N in STM-N: a relatively large number of check bits that must be created in the section overhead degrades (1) the transmission efficiency and (2) check bit generation and decoding takes time. To solve these problems, the STM-N signal error correction encoding/decoding method and STM-N signal error correction encoding/decoding circuits have been requested which can correct transmission errors simply by generating an extremely small number of check bits (error correction code) even when the value of N in STM-N becomes larger.